As a radial bearing equipped on a connecting device of a construction machine which operates under severer lubricating conditions such as high bearing stress, low-speed sliding and oscillating, a steel bearing of which at least the sliding surface is carburized or induction hardened is commonly used. Especially, in order to prevent occurrence of abnormal noise and improve seizing resistance at sliding, the steel bearing having the sliding surface on which plural of oil sumps such as lubrication grooves or dimples are formed has been used.
Recently, in view of the demand for easy maintenance, in order to extend a lubricating interval of the bearing, a high strength sintered oil retaining bearing; an oil retaining bearing in which a cylindrical high-strength brass material is subjected to a hole drilling and the formed holes are filled with porous graphite; and a self-lubricating sliding material in which a solid lubricant such as graphite, MoS2, WS2 and the like is dispersed in a sintered sliding material have been widely put to practical use.
As an exemplary self-lubricating copper based sliding material, a material in which a copper based sintered material containing a solid lubricant such as graphite, MoS2, WS2 and the like dispersed in the parent phase thereof is hot-pressed (SL alloy, manufactured by Toshiba Tungaloy Co., Ltd.) is given.
A ferrous sintered alloy having a structure in which free graphite of 1 to 5% is dispersed in a ferrous base which forms a solid solution with carbon of 1.2 wt % or less has excellent sliding performance. And, a sintered alloy for sliding material is known in which graphite particles are subjected to a suitable silver plating to form an iron-copper hardened layer 5 to 10 μm in thickness around free graphite in a ferrous sintered alloy and to disperse graphite blended with a raw powder as the free graphite without diffusing in the base (for example, referring to patent literature 1).
The above sintered alloy for sliding material has good initial sliding performance; however, has a serious problem in which abrasion loss increases remarkably with increase in hour of use. From the viewpoint, in the patent literature 1, a graphite powder blended with a raw powder is subjected to a copper plating so as to have a suitable thickness and a sintering temperature is strictly selected such that the copper plate layer does not disperse in and form a solid solution with ferrous particles so as to form an iron-copper hardened layer around the graphite particle. However, since the thickness of the copper plate layer is constrained and the sintering temperature is restricted to low temperatures so that the copper plate layer will not melt, sufficient sintering strength required to be used under high bearing stress whereby sufficient abrasion resistance and coefficient of friction can not be obtained.
A high strength self-lubricating sintered sliding material improved in abrasion resistance has been known (for example, referring to Patent literature 2). In the self-lubricating sintered sliding material, a mixed powder of a ferrous metal powder having a grain size of 45 μm or less and a solid lubricant granulated to have a grain size of 0.03 to 1 mm, such as graphite and MoS2, in an amount of 10 to 80% by volume is formed and sintered at 1050° C. in order to reduce association of the granulated solid lubricant. And, a copper alloy based infiltrating agent is infiltrated at the sintering in order to reduce concentrating of stress on the granulated solid lubricant.
The above self-lubricating sintered sliding material, however, does not have sufficient sintering strength because the sintering temperature is restricted to 1050° C. in order to prevent a reaction of the granulated graphite with the ferrous base at the sintering. And, a local penetration easily occurs on the ferrous base between the granulated graphite because of long particle spacing of the solid lubricant. And, the infiltration of the copper alloy based infiltrating agent leads to close pores of the sliding material, and, therefore, to inhibit lubricating performance of the retained lubricating oil. As the results, sufficient abrasion resistance and seizing resistance cannot be obtained.
Further, a slipping bearing has been known, in which a ferrous sintered alloy layer comprising a mixed powder of copper powder of 10 to 30 wt %, graphite powder of 0.1 to 6.5 wt %, molybdenum disulfide powder of 0.1 to 7.0 wt % and rest of iron is sinter-bonded to a steel back metal (for example, referring to Patent literature 3).
The above slipping bearing, however, has not sufficient sintering strength because of restricted sintering temperatures. The Patent literature 3 shows a method for adding the solid lubricant and an amount of the lubricant mainly, however, little research about a metal phase (ferrous) base excellent in seizing resistance.
One of oil retaining slipping bearings available for use under high load and having no solid lubricant dispersed therein is produced in such a manner that a compact of mixed powder prepared by blending atomized ferrous powder, copper powder or copper alloy powder, graphite powder, various types of high speed steel powder, ferromolybdenum powder and copper alloy powder (KOBAMEETO, manufactured by CABOT Supermetals K.K.) is sintered at temperatures at which the copper powder or copper alloy powder is melted and then cooled. This cooling process leads to precipitate copper phase or copper alloy phase which is dispersed in and forms a solid solution with the ferrous base. As the results, copper particles or copper alloy particles are dispersed in the iron-carbon alloy base in which martensite exists. And, an abrasion resistant ferrous sintered alloy for oil retaining bearing is known, which contains copper of 7 to 30 wt %, has alloy particle having a specific composition as a harder phase than the iron-carbon alloy base in an amount of 5 to 30 wt % dispersed therein and has porosity of 8 to 30% by volume (for example, referring to Patent literature 4). In the abrasion resistant sintered alloy for oil retaining bearing, a large amount of copper powder or copper alloy powder is blended as raw powder for the following purposes: (1) outflow pores needed for retaining oil are formed by melting copper powder or copper alloy powder at sintering; (2) soft copper particles are dispersed in a martensite phase for improvement in conformability; and (3) the above alloy particle harder than the martensite of the base is dispersed so as to reduce plastic deformation of the base and also reduce load applied on the base at slipping sliding, whereby excellent abrasion resistance can be obtained even under high pressure.
As the alloy particles, there are given; (1) ferrous base alloy particles (high speed steel powder particles) containing C of 0.6 to 1.7 wt %, Cr of 3 to 5 wt %, W of 1 to 20 wt % and V of 0.5 to 6 wt %; (2) ferrous base alloy particles (high speed steel powder particles containing Mo and Co) containing C of 0.6 to 1.7 wt %, Cr of 3 to 5 wt %, W of 1 to 20 wt %, V of 0.5 to 6 wt % and at least one element of Mo and Co of 20 wt % or less; (3) Mo—Fe particles (ferromolybdenum) containing Mo of 55 to 70 wt %; and (4) copper base alloy particles (heat resistant and abrasion resistant alloy particles for a build up spraying, KOBAMEETO manufactured by CABOT Supermetals K.K.) containing Cr of 5 to 15 wt %, Mo of 20 to 40 wt % and Si of 1 to 5 wt % (for example, referring to Patent literature 4).
The above oil retaining slipping bearing has problems of insufficient seizing resistance and high coefficient of friction for the bushing equipped on a construction machine operated under sever sliding conditions (high bearing stress and low oscillating) because of the alloy particle dispersed in an amount of 5 to 30 wt % therein.    Patent literature 1; Japanese Patent Publication No. S58-1 57951;    Patent literature 2 Japanese Patent Publication No. H4-254 556;    Patent literature 3; Japanese Patent No. 3168538; and    Patent literature 4; Japanese Patent Publication No. H8-10 9450.